It is well known by an artisan in the radar waveform design art that increased radar target resolution can be achieved by transmitting a linearly or non-linearly stepped FM signal pulse (toward a target) and processing its received echo by passing it through a network which processes linear (or non-linear) delay characteristics, with respect to frequency, so as to compress the received energy into a narrow high amplitude time pulse. Environments do exist, however, for radar operation wherein narrow bandwidth interference sources are active at frequencies within the same bandwidth as that of the stepped FM pulse. These interferences are received with the desired echo, generally at much larger amplitudes than the echo. Thus, when a combination of the received interference(s) and desired signal is processed by the linear delay network, the resulting compressed time pulse may become distorted, so much so as to become unrecognizable and therefore undetectable, due to the interferences.
To correct this problem, a current method involves the implementation of a narrow band elimination filter in the radar receiver prior to pulse recompression so that the unwanted interference frequencies are eliminated prior to passing the received echo through the linear delay network. Less perturbation to the ideal echo results and the received compressed pulse is restored to a shape which at least is detectable, rather than being totally unrecognizable.
Yet the use of band elimination filter in the frequency spectrum of the stepped FM pulse also results in the removal of energy from the desired signal. This leads to an intolerable decrease in the amplitude of the desired echo, thereby leading to a situation where the radar receiver often fails to detect these weak echoes.